Blown-film innovator continues to reach new heights with drives

Plastic blown-film is an integral part of our everyday lives--protecting food, products, and ourselves from contaminants and damage. It is used for everything from saran wrap to protective tarps that shield crops from frost. The process for creating such products is intense, each film type has its own particular resin compound characteristics and specific production requirements.

At Brampton Engineering (Brampton, Ontario) we use massive, four-story machines to produce blown film. These machines rely on advanced automated control systems to accommodate the various film production needs, whether it's high speeds for thin-film gauge applications or slower speeds for heavier-gauge films.

One of the most important factors in producing quality film for our customers is achieving precise motor speed and torque control throughout the process--this reduces gauge variations and film imperfections. A typical operation throws away 8 to 10% of produced film.

If proper tension control is present during the cooling process--when the film is in its most elastic state--we can reduce imperfections. For example, we could aim for a 2-ft wide sheet of plastic, but if our tension is off and we pull too hard or too fast through the winder, the plastic may end up being drawn down to 18 inches at the end of the process, creating as much as 1,000 lbs of scrap per hour.

The original brushless dc drives we were using in the machines provided only speed control. To improve tension control and reduce scrap we replaced them with advanced variable-speed ac drive technology, which controls the extruders, air blowers, take-off equipment, nips, and winders. However, the ac drives tended to lose speed regulation in the lower speed ranges. And, if speed varied more than 1/2% on the extruders, fluctuations in the flow would cause imperfections. Again, more waste. We needed more-reliable speed regulation throughout the entire speed range.

The solution was Allen-Bradley 1336 FORCE(TM) ac drives, which incorporate Force Technology(TM), a multi-patented, field-oriented control technology which separates and independently controls torque- and flux-producing current components. By controlling torque directly, the drives exceeded the performance capabilities of dc motors and drives we had used in the past.

This application uses seventeen 1336 FORCE drives, with 0.5 to 400 hp to control the extruders, winders, and nips installed on a blown-film line. Three more Allen-Bradley 1336 PLUS(TM) variable-speed drives provide constant speed control of the air blowers and cooling devices.

By placing the drives where speed and torque control is most critical, such as on the upper nip to control tension, we were able to achieve accurate speed regulation and eliminate the risk of the film's thickness changing as it is pulled off the die.

Using these drives in conjunction with our Integrated Temperature and Line Yield Control System (ITALYCS), hard wiring to the drive--for control of speed reference, speed feedback, load feedback, and discrete start and stop--was eliminated. The ITALYCS system, based on Allen-Bradley

PLC-5(reg) programmable controller communicating over FLEX I/O(TM) and ControlNet(TM), controls each stage of the line and eliminates the multiple controllers used by conventional lines. This also allowed us to tie the drives into the central control system.

The drives also simplify and reduce diagnostics and troubleshooting. AllenBradley DriveTools(TM) software, a comprehensive medium library, allows us to monitor hundreds of different parameters--this wasn't available on previous analog drives.

More importantly, we can achieve a precise level of speed and torque control necessary to produce high-quality blown film, while reducing scrap. Beyond that, we provide customers with reduced installation, maintenance, and equipment costs.

DESIGN OBJECTIVE

To produce the highest quality film possible, while greatly reducing the amount of product scrapped due to imperfections.

CHALLENGES

Identifying a drive that: Provides precise motor speed and torque control for production of quality film ranging from 12 to 250 microns thick, reduces overhead expenses, and decrease the percentage of film thrown away (currently 8 to 10%).

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